Internal Structure Of The Metal Engineering Essay

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Metal work is the use of metals to produce parts. Plastic deformation is when the metal is taken above its elastic limit and therefore will not return it its original form; this can be done by applying pressure or heat to produce parts. Sheet metal forming is a type of plastic deformation. Sheet metal processes are widely used in the automotive industry because of its ability to make complex parts as well as the accuracy of the process. There are many types of sheet metal forming such as bending, rolling or punching to name a few. It involves taking a piece of metal and putting them through high pressures to form them into thin flat pieces or even directly into parts [1]. As we can see from figure1, shows how sheet metals are produced. It is taken above its elastic limit and depending on its properties it can remain plastically deformed, this is because of the internal structure of the metal.

Figure 1[3]

Returns to original shape

Elastically deformed

Small force

Large force

Plastically deformed

Figure 2 [2]

In a metal the atoms are metallically bonded so they can go over each other and if enough stress is applied then the metal will plastically deform as shown in figure 2 [2].

The automotive industry uses metal working to produce parts such as car body panels. They must decide what the best process is to produce car body panels which will be lightweight as well as maintaining the safety of the passengers inside if it were to be in a collision. The manufacturer must also consider how cost effective the process is, the quality of output and whether or not it can produce the type of parts required. Car body parts are mainly made from steel but some manufacturers use aluminium [4][5].


It is unknown when metalworking began but it is said that the Egyptians were the first to start with gold [6]. As gold is found naturally on the earth and needs little or no refining so all that would be needed is a hammer and anvil which was most likely made from stone. Isaac Asimov said that gold is has properties such as low oxidisation level which means it is found pure whereas other metals are found as ores therefore require heating. Gold is malleable so can be easily shaped into objects. As man discovered fire they found that melting certain rocks, they could form objects with certain properties [7]. Artefacts such as necklaces or rings have been discovered. As time went on man discovered more and more elements and began mixing them together to make alloys and the first two metals mixed together were tin and copper to produce bronze the first alloy. This advancement in metalworking led to objects such as weapons and tools being made. As more metals were found the most fundamental metal was discovered, iron. This led to further alloys such as steel. With these new metals came their subsequent properties, which meant new methods were required to be able to work with the metal. This led to the use of forges which would heat metals making them more malleable at which point the blacksmith would shape the metal using an anvil and a hammer. Then came the modern manufacturing methods using machines to produce complex components [6] [7].

Classic Methods


Figure 3

Figure 4





Bar and wire drawing




Deep drawing



Drawing involves passing a metal through a die opening by either punching it as shown in figure 3 or pulling as shown in figure 4 which is also known as extrusion. This puts the metal under tensile and compressive forces causing the metal to be drawn out into shapes or wires. This process is mostly performed cold but can be performed hot to reduce friction and to make the material more ductile and malleable. Drawing also has ability to draw a range of different materials from aluminium to steel. Drawing is used in the automotive industry to produce car parts [8][9][10]. The advantages of drawing are that it can produce a high volume of parts economically. This is a good feature for the car manufacturers to have because it means they can save time and money on production. As drawing uses a single piece of metal to form parts, there are less stress points this means that the part will be stronger. However, drawing has the disadvantage that it cannot be used to produce complex shapes. This is a significant disadvantage to the car manufacturers as it limits what they can produce on the machines. In addition, it can sometimes produce defects such as wrinkling, tearing, earring and surface scratches which means that the part must be either discarded or further machining has to be done [9][10][11].


Rolling started out in the 14th century in which metals such as gold and silver were rolled manually, a rolling mill was designed by Leonardo Da Vinci but it was doubtful that it was ever built. In the 17th century cold rolling of lead and tin had developed on manual rolling mills but it wasn't till the 18th century that hot rolling had started and this was being done mainly in the European countries. As rolling requires a lot of energy due to the immense forces the rollers and the metal undergo, water wheels were used prior to the 18th century then came steam powered mills and now in modern manufacturing facilities electric motors are used [12].

Figure 5

Figure 6




Rolling uses compressive forces to reduce the thicknesses of metals [13]. Figure 5 shows how rolling is used to flatten metal, flat rolling, but figure 6 shows how with different roller configurations it can be used to produce parts which is called shape rolling[12]. Metal is squeezed between two or more rollers causing the metal to plastically deform. Rolling can be performed hot or cold or both. Hot rolling is mainly used for sheet metal as thicknesses can be reduced easier compared to cold rolling which would require the metal to be worked on a few times. But in some cases hot rolling is initially used then cold to increase the metals strength and hardness[12][13]. The advantages of rolling are that if the metal is hot rolled then the metal is less likely to contain residual stresses. This maintains the strength of the metal which is essential for safety in a car. It also produces work with high surface finishes and has high dimensional tolerances making it very precise and in can also be used in conjunction with other sheet metal processes [13][14]. This allows car manufacturers to maintain quality in their cars. On the other hand, the rollers can heat up and deform due to the high amounts of friction between the metal and the roller. If the process is performed hot this can also affect the rollers as well as the metal reacting with the air causing a layer of oxidisation to be produced [12][13][14]. This means that further processes must be used to clean the metal to reveal its original surface. This increases the cost of manufacturing for the manufacturer.


Spinning is a form of sheet metalworking which involves the work piece being clamped between the mandrel and a clamp, it is then spun at high speeds at which point a roller rolls with considerable force along the piece and forms it according to the shape of the mandrel as shown in figure 7.

The first sign of metal spinning ever being used was during the middle ages. At this time the spinning process was done by hand by two people, one would spin a wheel connected to a lathe, and then using a stick the other person would form the metal. Water wheels and later steam power was used to power the mandrel, but it wasn't until the electric motor was added that spinning came into its own. However, there was still room for development; hydraulic power and a roller were added to the system. But the system was not completed as for higher production rates highly skilled craftsmen were necessary, this led to the use of computers in spinning, CNC machines are now used in spinning [16].

Spinning allows manufacturers to form objects such as cups, cylinders or tubes. The main advantage of spinning is that it allows the manufacturer to produce parts that have large diameters at a cheaper cost compared to other processes such as where dies are used. Another advantage is that very little equipment is needed which lowers production and maintenance costs. The mandrel can be made from softer materials to that of metal such as wood which is easier to form shapes and it also lowers the cost of the tool compared to processes which require punch and dies. It can produce high quantities of work. It also produces less waste compared to other methods. However, if the metal breaks or cracks it has to be discarded as it is not worthwhile to the manufacturer to repair it. So in some aspects it is quite a wasteful process. [17][18]







Figure 7


Cutting is where sheet metal is cut to the required size by removing the excess allowing it to be further machined. There are a variety of methods that can be used to cut sheet metal such as blanking, punching and shearing [6].


Blanking is a cutting process where the required part is cut from the sheet metal. Some of the materials commonly used in blanking are aluminium, brass and stainless steel [19]. The metal is balanced on two dies. A punch which is slightly smaller than the gap between the dies punches the sheet metal. The metal undergoes plastic deformation as the punch hits the metal, once it has penetrated deep enough the metal is taken to breaking point. The metal eventually breaks producing the required part [20][21]. The advantages of blanking are it is an economical method of producing parts in medium to high production processes. It can also produce holes of different shapes. It is also environmentally friendly because the scrap metal can be recycled. The disadvantages are that it produces rough edges which would need further machining [19][20][21].


Discarded Part

Worked Part


Discarded Part

Figure 9

Figure 8

Worked Part


Punching works by cutting out parts which are not required. It works the same way blanking does but it creates holes. The metal is placed on top of two dies which are separated slightly bigger than the punch leaving a clearance. The sheet metal is punched but as it is hit plastic deformation occurs initially, penetration happens when the punch is about a third of the way down taking the metal to its ultimate tensile strength. It is then taken beyond its UTS causing the metal to break. The punched piece is scraped and is likely to be recycled to be used again making the process environmentally friendly. As with blanking it can produce cuts of varying shapes, it is also an economical method of producing parts. The disadvantage is that it also produces rough edges which would require further machining [21][22][23].


Shearing involves cutting sheet metal into size by placing it between two sharp edges as shown in figure 10. There are many types of shearers such as guillotines, tin snips or bench shears. They are all based on the same principle but each has its own advantages and disadvantages. The metal needed to be cut is placed on the die, the blade then comes down with force, initially plastic deformation occurs but as it gets further down the metal, it begins to penetrate it. The metal then fractures and breaks apart. The main advantages are that it can cut metal quickly and efficiently giving it a high production rate which is good for car manufacturers which mass produce cars. However, the cuts can leave rough edges; therefore work needs further machining to complete the process. The blades will wear out and will need to be replaced [24] [25] [26].





Figure 10


Start prior to plastic deformation



Bending is a sheet metalworking process where a piece of metal is bent into a required shape. Typical forms of bending are V-shape, U-shape or wiping dies to create flanges or usually 90Ëš bends [27]. Prior to bending metal, calculations must be performed to take into consideration like bending allowance and springback; this is because of the forces acting on the metal during bending. During bending, the inside of the metal is in compression and it is in tension on the outside of the metal, as shown in figure 12 [28][29]. Too little force would cause the metal to bend to the required shape but it would then springback producing a part which isn't at the required bend, too much force could fracture the metal [33]. Bending is used in the automotive industry to produce parts such as car body panels. The advantage of bending is that it is a cheap method of metal forming as not that much equipment is required. This makes it a low maintenance method of manufacturing. However, V dies are more simple therefore less expensive but used for lower production parts, whereas wiping die set ups are more complicated and expensive so it is used for higher production parts. The disadvantages of bending are that if the metal is not bent to the required shape or it has been bent in the wrong place then the material has to be scraped because the metal will be weaker if it is unbent and bent again, since this method is mainly used for parts made from thin sheet metal. In addition, the process is mainly used for simple designs as opposed to complex ones because it is far more expensive to do complex shapes on a bending machine compared to other processes [28][29][30].

V-punch male die





V -female die

Figure 11

Figure 12


Forging is said to have started as early as 5000 B.C.E [31] and "is one of the oldest metalworking processes" [32]. "There is also evidence that forging was used in ancient Egypt, Greece, Persia, India, China and Japan to make weapons, jewellery, and a variety of implements."[31]. It was highly reputable being a specialist in forging during these times. The process is more or less the same; an impression die in conjunction with a hammer would be used to form the metal into the required shapes. More and more complex impressions were developed as time went on. It wasn't until around the 18th to the 19th century that forging changed and became more automated. Now it is used to make components in most of the modern machines such as in the automotive or aerospace industry [31].

Forging uses the principle of reducing the strength of the metal and increasing ductility and malleability of the metal by heating the metal. However, it can also be performed cold. In the automotive industry forging is mostly performed cold [33]. There are two type of forging machines, one is called a forging hammer because it uses impact loads to from the metal, and the other is a forging press which works by steadily applying pressure. There are three types of forging, open die, impression die and flashless forging. Open die is when the metal is placed between two, sometimes flat, dies, compressing the metal. Impression die is when the metal is placed between two dies which are custom made to make a specific shape, the molten metal is then forced to make the shape of the die, some of the metal flows outside of the die causing a flash which is extra metal, this will need to be trimmed. Flashless forging is basically impression die forging but a flash is not produced because the amount of metal used is controlled and is totally contained within the die [31][32]. Some of the materials involved in this process are aluminium, copper and mild steel [34]. The advantages of forging are that when performed cold the strength of metal is increased. However, it makes it difficult for further machining to occur because cold forging cause the metal to harden. On the other hand, forging can manufacture very complex shapes. The grain structure follows the shape so it increases the strength of the component [35]. The disadvantage of forging is that processes such as impression or flashless have custom dies which are expensive because they have to be extra resilient due to the temperatures and forces involved[33].

Alternative methods

Water only and Abrasive jet cutting


Sheet metal

Sheet metal


Abrasive material



Figure 13

Figure 14

Abrasive Water Jet cutting Water only Jet cutting

This process allows for a range of materials to be cut, 'water only' cuts more of the softer materials such as leather, wood and carpet whereas 'abrasive jets' cut harder materials such as carbon steel, titanium and iron [36]. Its ability to cut a wide range of materials makes it an ideal candidate for the automotive industry as it can be used to cut pieces for the interior as well as car body panels. Some of the more advanced machines have 5 axis therefore they can produce very complex shapes. The main advantage of this process, as mentioned previously is that it can cut a wide range of materials [42][43]. It also produces smooth cuts so additional finishing processes are not required. In addition, as no heat is produced there is no heat affected zone so the the strength of the metal is maintained [37]. This process can also cut very complex shapes and it is environmentally friendly as the water used can be recycled. These are important advantages to the car manufacturer as maintaining strength improves the safety aspect of the vehicle compared to processes such as plasma or laser cutting and car manufacturers are constantly trying to find ways of making their vehicles environmentally friendly so why not make the manufacturing process just as "green". However, it takes a lot longer to produce parts as opposed to other processes especially if a smooth cut surface is required [38][39][40][41]. This is a major disadvantage for car manufacturers as it means that it will take longer to produce cars which could be costly to the manufacturer. The thickness that it can cut is dependant on the material to be cut which limits what it can do [38][39][43].

Hydroforming (Sheet)


PistonHydroforming is a process similar to drawing and bending, however, this method uses highly pressurised fluid as opposed to a punch or die. To hydroform a piece of sheet metal it is placed on top of a mould of specified shape. It is then closed to create a chamber to prevent air leaking in and fluid leaking out . The pressurised fluid is then forced into the chamber by a hydraulic piston. The fluid applies this pressure to the sheet metal until it is the same as the mould [44]. One of the main advantages of hydroforming is that a male die is not required as the fluid effectively becomes the male die. This means that the manufacturer will save money. Another advantage is that the pressure is exerted equally over the length of the metal so it will keep the thickness of the metal constant,forming shapes of higher strength than deep drawn shapes, and reduces the risk of the metal fracturing as opposed to deep drawing where the pressure is mostly concentrated at a single area [44][45]. The disadvantage of hydroforming is that softer materials can end up sticking to the female die, the material tends to accumulate on the mould, this affects the following parts which means that the die has to be changed which can be costly [46].

Pressurised fluid




Figure 15

Electrohydaulic forming

Power SupplyThis is a form of high energy rate forming (HERF) in which short bursts of energy are used to deform the sheet metal into the mould. The process works by submerging the sheet metal in a transmission fluid over the die. A vacuum is created between the sheet metal and die which helps to form the required shape. Two electrodes are put in the transmission fluid above the sheet metal. These are connected to a high capacity capacitor and a power supply. The gap between the electrodes allow the electrical energy to be discharged as shock waves at a high velocity forcing the sheet metal into the die, as shown in figure 16. The advantages of this process are similar to that of hydroforming in that there is only a female die so the manufacturer saves on the cost of a male die. The energies involved to produce the shock wave are smaller compared with other HERF processes such as explosive forming. The process is quick so a lot of parts can be produced in a short amount of time. However, a big disadvantage is that because of the small amounts of energy the process uses it can only make smaller components.

Transmission Fluid



Shock Waves

Vacuum Line





Figure 16

Laser Cutting

Laser stands for light amplification by stimulated emission of radiation. It uses light and heat to cut through various materials.

Laser cutting was first used in 1965 to drill holes into diamonds. A laser assisted oxygen jet was pioneered by the British in 1967 to cut metals. It wasn't until the 70's that it got used to cut metal for aircraft production. The laser was modified to use carbon dioxide as opposed to oxygen because they could cut non-metals [49].

The laser cutting process works by stimulating a lasing medium which is contained in the laser, this is full of atoms; these atoms are taken to an exited state. Specifically in the atoms the electrons are taken to higher energy states, taking them out of their original orbital movement. The electron then releases this energy in the form of a photon (light energy) and the electron returns to its original state and orbit. The photon has a particular wavelength; electrons in the same state will release photons of the same wavelength. Mirrors at either end of the lasing medium reflect the wavelengths back and forth down the lasing medium. The amount of photons with identical wavelengths builds up. One of the mirrors is semi-silvered causing some of the photons to be reflected and some let out, the ones that make it out are the cause of the laser light and gives the ability to cut metal if the beam is strong enough [50].

The advantages of laser cutting are it is more accurate compared to more conventional methods such as shearing. It can also cut tougher materials easier than the mechanical methods. It leaves a smoother cut as opposed to other processes reducing the need for further machining. For the automotive industry it means that they can make cuts which are more accurate and of higher quality. However, laser cutting uses a lot of power but this is dependant on the material and the thickness being cut [49]. Lower quality materials which contain impurities or inconsistencies reduce the laser's precision giving jagged cuts [42]. In addition, because the laser heats the metal it leaves a small heat affected zone, this leaves weak point in the metal [49].


It is evident that there are many manufacturing processes available to the automotive industry, it is therefore vital that the manufacturer chooses the correct sheet metal forming technique in terms of material, design and cost.

All processes discussed are sheet metal forming processes, however, some can do what others can't, for example a water jet cutter can't form the metal into shapes like a hydroforming machine can, and vice versa. Therefore, the automotive industry can't simply pick one process to do all the work, a system of machines are needed.

In terms of cutting, I think that the best method for the automotive industry would be the abrasive water jet cutter. Comparing it to classical forms of cutting such as punching, blanking, shearing and more modern forms such as laser cutting, it comes out as the best method because where the classic methods have the disadvantage of leaving rough edges on cuts meaning that they need further machining, abrasive water cutting produces smooth cuts. In addition, shearing has the added disadvantage of the blades wearing out, but abrasive water cutters are of lower maintenance. Laser cutting produces small heat affected zones because the metal is heated to cut it but with abrasive water jet cutting no heat is involved meaning it does not produce them and because no heat is used it consumes less power. The only disadvantage is that the production rate is slow for high quality cuts.

I think for metal forming, the best methods are forging, hydroforming and rolling. I think it is necessary to have all three because where one fails to meet the requirement it is possible with the other. For example on comparison of hydroforming to the other metal forming process, one of the disadvantages of hydroforming is that softer materials which are to be hydroformed can stick to the die. However, if the manufacturer can forge or roll as well then the softer material can be used on these processes instead, and harder materials can be used in hydroforming where the rollers may not be able to perform the operation. Processes such as drawing are unable to produce complex shapes this is also a disadvantage of spinning and electrohydraulic forming. However, spinning does have the ability to produce parts of large diameters compared to forging, hydroforming and rolling. On the other hand, with the correct roller configuration, it is possible to get to diameters such as the ones that can be produced on spinning but it would be costly. Other advantages of the chosen processes as opposed to other processes are that they can produce components of higher strengths. The overall disadvantage of the processes is that they can be costly, such as the die for hydroforming and forging can be expensive depending on its shape. However, I think that these processes have the ability to perform the same operations just as well as the other processes, that's why I think they are the best methods for metal forming.

To summarise, I think that in the automotive industry there is no standard set of machines that are used to make all the different types of cars by all the different manufacturers. Therefore, there is no common method for manufacturing among the automotive industry. I think that each manufacturer will have different manufacturing process depending on what type of cars they manufacture. For example manufacturers such as Rolls -Royce, build their cars to the highest quality so they will use sheet metalworking techniques such as water jet cutting or hydroforming which is costly but keeps their quality and this is reflected in the price of their cars. Whereas, manufacturers such as Suzuki are more likely to use techniques such as shearing or drawing where the quality isn't as high as but the production rate is, this allows Suzuki to give their customers lower prices. It all depends on what market the car manufacturer is aiming for like Rolls - Royce would be the niche market whereas Suzuki is more mass market.


Cars have been manufactured for over 100 years now, and the manufacturing process has changed drastically. With the ability to make stronger materials and improvements in technology, manufacturers are able to produce more complex products than ever before. [52][54]

In recent times the automotive industry has become more environmentally aware. This has led to the development of more efficient cars. A recent development has been the use of carbon fibre in cars. Carbon fibre dates back to the late 19th century where it was used by Thomas Edison as filaments for light bulbs. It was not until the 1970's when experiments were conducted to find alternative materials that it was commercialised [52][54]. Carbon fibre is 10 times stronger than steel and half the weight giving it a very high strength to weight ratio [51]; this helps the car to be more fuel efficient as well as improving safety. It is widely used in the racing industry, but recently started being used in super cars such as the Koenigsegg CCXR edition which has a carbon fibre chassis [55]. It isn't really used in mass production cars as the cost of carbon fibre is quite high. But as it gets cheaper it is likely to replace the current steel and aluminium frames. This new material uses a completely new manufacturing process, it goes through five stages: spinning, stabilizing, carbonizing, treating the surface and sizing [53]. Therefore, the classic methods and more modern methods may become redundant as carbon fibre becomes more popular among manufacturers. But this will bring in new methods of manufacturing possibly based on the classic and current methods, and I think it will be a slow move to carbon fibre manufacturing.

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